EtherCAT basics

Processing on the fly

An EtherCAT master sends one Ethernet frame (EtherType 0x88A4) that passes through all slaves in a logical ring. Each slave reads its outputs and inserts its inputs while the frame passes, with nanosecond-level delay, and the last slave sends the frame back. One frame can serve hundreds of devices, so cycle times of 100 µs and below are possible with standard Ethernet hardware in the master.

Topology

Line, tree or star using junctions; the frame always runs through every device. Ports are typically labelled IN/OUT — swapping them changes the order of devices and confuses the configuration. Cable redundancy closes the line back to a second master port.

State machine

If a transition fails, the slave reports an AL status code (e.g. 0x001B sync manager watchdog, 0x0011 invalid requested state change); the Diagnostic Database contains the Beckhoff list.

Device description and mailbox protocols

Slaves are described by ESI files (EtherCAT Slave Information, XML). Mailbox protocols carry acyclic data: CoE (CANopen over EtherCAT — the CANopen object dictionary and SDOs, CiA 402 drive profile), FoE (file access, firmware update), EoE (tunnelled Ethernet), SoE (Sercos profile).

Distributed clocks

Slaves can synchronise their local clocks to a reference clock with sub-microsecond accuracy, so outputs switch and inputs latch at the same moment across the machine — essential for motion control.